The present invention relates to ink jet printing, and in particular to an improved deflection electrode assembly for a continuous ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging electrode where the individual drops are charged to selected voltages. The drops then pass through a transverse electric field (deflection field) provided across a pair of deflection electrodes. Each drop is deflected by an amount that depends on its respective charge. If a drop is uncharged, it will pass through the deflection electrodes without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate moves past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
Better control over drop placement, and hence print quality, can be achieved by maintaining the highest possible deflection field strength at all times. In this respect, it is known to bend or angle the positive (high voltage) deflection electrode to generally conform the deflection electrode to the path of the charged drops. The strength of the deflection field can also be increased by moving the deflection electrodes closer together. However, moving the electrodes closer together increases the tendency for arcing between the deflection electrodes.
Control over drop placement can also be improved by minimizing the aerodynamic effects and the drop-to-drop charge interaction effects. Both are reduced by shortening the flight distance between the charging electrode and the substrate. However, moving deflection electrodes too close to the charging electrode increases the likelihood of arcing between the high voltage deflection electrode and the charging electrode.
In order to reduce arcing, it is known to provide insulating material on the high voltage deflection electrode, and in some instances additionally on the low voltage deflection electrode. Certain inks, including pigmented inks, have a tendency to create micro-satellite drops. It has been found that the micro-satellite drops tend to accumulate on the insulating material. The accumulation of the charged micro-satellite drops on the insulating material decreases the strength of the deflection field, thereby decreasing drop deflection and print quality. When this occurs, the printer must be shut down so the ink build up can be removed from the insulating material. As will be appreciated, the need to shut down the printer is highly undesirable in many applications.